Title:
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Application of the finite element method to numerical weather prediction
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The finite element method is applied to wave propagation and
advection problems often used to develop models for numerical weather
prediction. For these problems the error analysis must distinguish
the error in representing the initial and final states from the error in
evolution, for instance the error in wave speed. The analysis indicates
that finite element methods can only compete with explicit finite
difference schemes if "super-convergence" can be obtained. For the
Galerkin method this means using splines on a regular or almost regular
mesh. This prediction is confirmed in actual tests. Because the finite
element approximation provides a complete field rather than gridpoint
values alone, the analysis based on it is more meaningful than a gridpoint
analysis, there is no "aliasing".
The linear spline Galerkin method is then applied to test problems.
The high accuracy for given resolution has to be balanced against the
extra work involved. For linear problems the results agree with theory.
The method is equivalent to an explicit fourth order method with 1.6 times
the linear resolution; the efficiency is comparable to explicit finite
difference schemes. The nonlinear problems solved include the shallow
Water equations in a channel and on a sphere. The finite element algorithm
is only competitive if the nonlinear terms involving derivatives are
evaluated in two stages. If this is done the integration on a sphere is
particularly successful. Artificial viscosity is necessary for realistic
problems, the accuracy of the finite element method allows more refined
forms of nonlinear viscosity to be used successfully.
In general the spline Galerkin method is competitive with explicit
finite difference schemes. For nonlinear problems it may give a fundamental
improvement, as the spectral method does. However, extreme care has to be
taken to get enough accuracy to balance the extra work.
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